Apparatus for measuring electrical characteristics



Jan. 14, 1958 J. L. REINARTZ 2,820,194

APPARATUS FOR MEASURING ELECTRICAL CHARACTERISTICS Filed Nov. 18, 1953INVENTOR 1751222 Z. fiez'aa'ziz;

BY w W M1464, +20% ATTORNEYS United States This invention relates toimprovements in measuring apparatus for accurately determining thecharacteristics of electrical elements, and more particularly toimproved electrical networks for comparing electrical elements ofunknown characteristics with similar electrical elements of knowncharacteristics. It is an object of the invention to provide anelectrical network or bridge circuit whereby comparison of circuitelements or components of known and unknown values may be rapidlyetfected, preferably at a speed commensurate with the speed ofproduction.

In the manufacture of circuit components, whether of capacitive,inductive, or resistive nature, it is essential to determine whether thevalue of the component falls within a range of values which can beemployed effectively; this is often of more importance than thedetermination of the exact value of the component. Furthermore,electrical components for use in high frequency circuits should betested at frequencies of the order to which they will be subjected inservice.

This is obviously true where the characteristic of primary importance iscapacitance or inductance, and is equally true in measuring theresistance of a conductive element in which, at high frequencies,current flow tends to follow the conductor surface. Thus an elementdesigned for high frequency use, found to have a desired value whentested at low frequency, will not necessarily function satisfactorily atthe high frequency encountered in service.

It is therefore a more specific object of the invention to provide ameasuring or comparing circuit in which the deviation of the signficantvalue of an electrical element from the optimum value at a frequency ofthe order to which it will be subjected in use may be quicklydetermined.

A further object of the invention is the provision of a bridge typeelectrical network comprising two generally equivalent circuits, onecircuit including an element of known characteristic and the othercircuit including an element of unknown characteristic, a source ofradio frequency energy applied across each of the circuits, a rectifierin each circuit, and a direct current meter to which the rectifiedcurrents from the two circuits are supplied in opposed relation, wherebythe difierential current flowing through the meter affords an indicationof the extent of deviation of the value of the element of unknowncharacteristic from that of the element of known characteristic.

Further objects and features of the invention will be apparent from thefollowing description, taken in connection with the accompanyingdrawing, in which the single figure is a diagrammatic representation ofa measuring circuit embodying the principles of the invention.

In order to facilitate an understanding of the invention, reference ismade to the illustrated embodiment and specific language is employed todescribed the same. It will, nevertheless, be understood that nolimitation of the scope of the invention isthereby intended, variousalterations and further modifications being contemplated such aswouldnormally occur to those skilled in the art to which theinventiodrelates.

' ;Th u$ teferring'to'the'diawing, 'there is shown a source c... t r

' atent O 2,820,194 Patented Jan. 14, 1958 ICC of radio frequency energy10, which may be of any conventional design, and is preferablyconstituted by a crystal controlled oscillator of 2 to 5 watt output ata frequency comparable to that which will be applied in use of theelement to be measured, for example 1 to 10 megacycles, the source beinggrounded at one side as indicated at 11. The signal generated by source10 is applied across each of two circuits, or of two arms of a circuit;an element or component 15 of known value, hereinafter referred to asthe standard element, is included in one arm and the element 16 ofsimilar design of which the value is unknown, is included in the otherarm.

The invention finds its principal application in measuring the value ofelements of which the significant characteristic is the capacity, and isespecially useful in measuring the interelectrode capacity in vacuumtubes and the like. Elements l5 and 16 are therefore represented ascondensers in the drawing, but it will be understood that inductors andresistors may similarly be measured or compared. Further, the testing orcomparing of units of more complex nature to determine the impedance,reactance or other characteristics thereof is contemplated.

in series with the standard element 15 in the first arm of the circuitis a resistor 18, a rectifier 19, and a condenser 20. In series with theelement 16 of unknown value in the second arm of the circuit is aresistor 24, a rectifier 25 and a condenser 26, corresponding elementsin the two arms preferably having identical values. By way of example,but not by limitation, resistors 18 and 24 may comprise 1000 ohm carbonor composition resistors of V2 watt rating, rectifiers 19 and 25 arepreferably germanium diodes, such as the 1N52 diode, or vacuum diodes ofthe 201C type, and condensers 20 and 26 may be 0.01 microfarad silvermica capacitors.

A meter 30, responsive to direct current, for instance a sensitivemicro-ammeter, preferably of the zero center type provided with a scalereading of 25 microamperes at each side of the zero point, has itsterminals connected through radio frequency chokes 31, 32, for examplechoke coils of the order of 2 /2 millihenry value, to the respectivearms of the measuring circuits, preferably at points 35 and 36intermediate the rectifier 19 and condenser 20 in the first arm, and therectifier 25 and condenser 26 in the second arm. The point 36 in thesecond arm is also connected through resistor 38 to the standard element15 in the first arm, and the point 35 is likewise connected throughresistor 39 to the element 16' Resistors 38 and 39 are of equalofunknown value. value, preferably the value of resistors 18 or 24.

It will be appreciated that when an element 16 to be measured ispositioned in the circuit as shown, and the source 10 is generating aradio frequency current of the order desired, the current flow willfollow the path including, in the one arm, the standard element 15,resistor 18, and rectifier 19, producing at 35 a pulsating directcurrent. Similarly, current will flow in the other arm through element16, resistor 24, and rectifier 25, producing at 36 a pulsating directcurrent. If the values of 15 and 16 are equal, the mean potentials at 35and 36 will be equal, and no current will flow in the meter 30, whichwill give a zero reading, indicating precise conformity of the unknownelement 16 with the standard element 15. If the value of 16 varies fromthe value of 15, the mean potential developed at 36 will correspondinglydiffer from that at 35, with the result that the meter 30 will bedeflected in one direction or the other, indicating the extent anddirection of the divergence in values of the elements 15 and 16.

The radio frequency chokes 31 and 32 prevent short circuiting of theradio frequency energy by the meter and protect the meter againstdamage. Capacitors 20 3 from the points 3.5 .and,36, .and .resistors,l8and 24 pr otect the rectifiers 19 and Zirespectively from the effects ofexcessive current flow. Resistors 38 and 39 complete paths for radiofrequenoyenergy :through the-condensers and the elements being compared.Accumulation er a residual charge :on condensers20 and 26frespectively-is prevented by continuous draining .:of .any icharg'ethrough resistors 38 and 39, so that elements 1610 .be measured may beintroduced :in the circuit inrapid succession: for the taking ofsuccessive readings.

Since it is usually sufiicient :merely to compare the unknown elementwith the .standard element, ifOl' the :purpose of 'determiningwhetherthe former 'has ta value fdeviating excessively from the value'oftheilattergtheamemr may initially be calibrated-by'selectingforstiietelement 15 one whichhas-the optimum value, andfeomparingwith itin succession elements =16 rhaving known values respectively'higlier andlowerjthan the toptirnum value, and-representing the limiting =valueswhich-may be'tolerated. The reading of the meter ineachlinsta'ncetisnoted, and the scale maybe marked tosshow theltwo Iextreme positions.Thereafter, on the testing :of :successive elements, those causing adeviation of the meter beyond .'the marking at either side may berejected, while those giving a less-deviation, and falling'within-ithezpredetermihedrrange of values, are acceptable 'for use.

While I pre'fertouse *frequenciestofithe-i-order to which the testedelement will The subjected in use, it will be appreciated that, ingeneral, the higher the-frequency of the source energizingthe.testcirciiiL the'greatEr the sensitivity of the measurement. Imay,therefore,:employ for the purpose frequencies much higher :than thesuggested range of "1 to "megacycles.

Having thus described the invention, .what is ?elaimed as new anddesired to be :secured by Letters Patent is:

1. In apparatus "for comparing "an electrical device of unknowncharacteristic with a simiiar electrical devi'c'e of knowncharacteristic, the combination with a source of high frequency energy,of a direct current-responsive meter, a pair of circuits, each cii'cuitincluding, in series with one of said devices, a-resistor,. a:diode, anda condenser, one ter'minalof each of'said c'onde'nsers rbeing directlyconnected to one'a'nother and said-diodes being poled so that thecommonly connected terminals'assume the same charge polarity, aninductiveimpedance'element connecting each of the meter terminals withone of the circuits at a point intermediate the "diode-and thecondenser, a second resistor connecting the last named point in each ofsaid circuits withapoint in the'other circuit intermediate theelectrical device'and thefirst'named-resistor, corresponding elements ineach of said circuits having comparable characteristics, and means forpressing energy from said source across said circuits.

2. In apparatus for comparing an electrical deviceof unknowncharacteristic with a similar device of known characteristic, thecombination with asource of radio frequency current, of a direct currentresponsive meter, a pair of circuits, each circuit including,'i'n serieswith one of said devices and in-the order named, -a'r'esistor, arectifier, and a condenser, one terminal of each of said condensersbeing directly connected'to one another and-said diodes being poled sothat'the commonly connected terminals assume the same charge'p'olarity,an inductive-impedance element connecting each of the meter terminalswith one of the circuits at apoint intermediate the rectifier and thecondenser, a secondresistor connecting the said point in e'aehof saidcircuitswitha"point inteimediate the electrical device and the firstresistor of the other of said circuits, corresponding elements in'eachof said circuits having comparable characteristicsf andEm'e'anS forimpressing current from said source "across said .circuits.

*3. In apparatus "for comparing an electrical device of unknowncharacteristic with "a "similar electrical 'device eenmwn'characteristic, the'eombinatien withasource of high frequency energy, ofa direct-current responsive meter, a pair of circuits, each circuitincluding, in series with one of said devices, a resistance device, adiode, and a condenser, one terminal of each of said condensers beingdirectly connected to one another and said diodes being .pqled 'so thatthe commonly connected terminals assume the same eharge polarity, aradio-frequency inductor connecting each of the meter terminals withone.

of the circuitsat a .point intermediate the diode and the condenser, aresistor connecting the last named pointin each or'said circuits with apoint in the other circuit intermediate the electrical device and theresistance device, corresponding elements in each of said circuitshaving comparable characteristics, and means for impressing energy fromsaid source across said circuits.

4. In apparatus for comparing an electrical device of unknowncharacteristic with :a similar electrical device of knowncharacteristic, the combination with a source of high :frequency energy,of a direct-current responsive meter, a pair of series circuits, eachcircuit including one of-said devices, a diode'and a condenser, oneterminal of each of--said-condensers .being directly connected to oneanother and said diodes being poled so that the commonly connectedterminals assume the same charge po- 1arity,-each of saidmeter terminalsbeing connected with one ofrthe circuits at a .point intermediate thediode and the-condenser,-an-impedance element connecting the last named{point in :each of said circuits with a point in the other circuitintermediate the electrical device and the rectifier, correspondingelements in each of said circuits having comparable characteristics, andmeans for impressing energy from said source across said circuits.

5. In apparatus for'comparing an electrical device of unknowncharacteristic witha-similar device of known characteristic, thecombination with a source of radio frequency-current, of adirect-current responsive meter, a -pair-ot' circuits, each-circuitincluding, in series with one of said devices and in the order named, aresistance device,arectifier anda condenser, one terminal of each ofsaid condensers being connected to one another and said rectifiers beingpoled so that the commonly connected terminals assume the same chargepolarity, an inductive impedance element connecting each of the meterterminals with one of the circuits at a point intermediate the rectifierand the condenser, a resistor connecting the said .point in each of saidcircuits with a point intermediate the-electrical'device and theresistance device of the other of said circuits, corresponding elementsin each of saidcircuits having comparable characteristics, and means'forimpressing current from said source across said circuits.

6. In apparatustorcomparing an electrical device of unknowncharacteristic with a similar device of known characteristic, thecombination with a source of radio frequency-current, of adirect-current responsive meter, a pair of series-circuits, each circuitincluding one of said devices, a'rectifier and a condenser, one terminalof each of-said-condensers being directly connected to one another andsaid rectifiers being poled so that the commonly connected terminalsassume the same charge polarity, each of said meter terminals beingconnected with one ofthe circuits at a-point intermediate the rectifierand the condenser,-means1for discharging said condensers including'animpedance'element connecting the saidpoint in each of said-circuits witha vpoint'intermediate the electricaldevice and-the rectifier of theother of said circuits,corresponding-elements in each of said circuitshaving comparable "characteristics, and means ,for impressingcurrentfrom said source across said circuits.

References-Cited in the file of thispateut UNITED S'.F.A="H$S PiA'PENTS2567,5 66 'Frisbie et'al. May 1"6,"- 195 0 2,508,446 Conant May 23, 19502,593,175 "Packard Apr. 15, 1-952

